A semiconductor device having a trench gate structure has been disclosed in, for example, JP 2005416822 A (hereafter referred to as Patent Document 1). This semiconductor device includes: an n-type source region; a p-type body region; an n-type drift region; an n-type drain, region; a trench; a gate electrode; and a p-type floating region. The source region is formed on a front surface side of a semiconductor substrate. The body region is formed under the source region. The drift region is formed under the body region. The drain region is formed under the drift region. The trench is formed through the source region and the body region from the front surface of the semiconductor substrate, and a bottom of the trench is positioned in the drift region. The gate electrode is accommodated in the trench in a state of being covered by an insulating film. The floating region is formed directly under the trench.
In the semiconductor device of Patent Document 1, while a gate voltage is off, a depletion layer spreads from a pn-junction part between the body region and the drift region, and a depletion layer spreads also from a pn-junction part between the floating region and the drift region. That is, since the semiconductor device in Patent Document 1 has the floating region depletion of the drift region is promoted, and achievement of higher withstand voltage between the source and the drain is thereby intended.